For a centrifugal unit, when a running load is reduced, inlet guide blades of a centrifugal compressor are closed gradually, and an air suction quantity is decreased. Suppose a passage area of a diffusion cavity remains unchanged, a flow velocity of air is decreased. When the flow velocity of the air cannot overcome a resistance loss of the diffusion cavity, an airflow will stop. Due to the reduction of kinetic energy of the air, converted pressure energy will be reduced accordingly. When an air pressure is smaller than an exhaust pressure, the airflow flows backwards, and surging is caused. For a single-stage centrifugal compressor, in order to prevent a centrifugal compressor from surging, a diffusion cavity adjustment device and an adjustment mechanism are often linked to act on an outlet of an impeller. When the running load is reduced, the flow velocity of the air can be increased by decreasing the passage area of the diffusion cavity, thereby effectively preventing the centrifugal compressor from surging. For a multi-stage centrifugal compressor, in order to improve the efficiency, an economizer generally exists between every two stages, and therefore an air supply opening exists. If a single movable diffusion cavity adjustment device is used for example, when the centrifugal compressor surges, a surging phenomenon will be improved only at a first stage having an adjustment function since only a single-stage impeller outlet diffusion cavity has the adjustment function while other stages do not have this function, and the surging phenomenon cannot be overcome at each of the other stages yet. If any stage of the multi-stage centrifugal compressor surges, normal working of an entire machine will be influenced, the centrifugal compressor is seriously destroyed, the performance of the centrifugal compressor obviously deteriorates, the noise is increased, vibration of the entire unit is greatly aggravated, and the running reliability of the unit is seriously influenced.